Field of the Invention
The present invention is concerned with inks for carrying carbon nanotubes for printing of conductive traces.
Description of the Prior Art
Carbon nanotube (CNT) coatings need to be applied by a variety of methods. Many printing techniques require viscous coatings, which necessitate that the CNTs be dispersed into some sort of medium. For example, screen printing requires a very high viscosity that is not easily achieved with the typical methods of dispersion for CNTs. Once the CNT coating has been printed, the medium carrying the CNTs has to be removable so that the unique optical and electronic properties of the CNTs are not diminished.
Formulations have been created using a low viscosity surfactant dispersion to apply the CNTs (via spray or rod drawdowns) to get the final desired properties. This method, however, requires a post-application step of washing the film to remove the surfactant so that the CNT properties are not diminished.
Another similar technology is the V2V™ process from Chasm Technologies, Inc. (Bielek, 2005). The V2V process is a process of dispersing the CNTs in a low viscosity medium (acid or amine solvents). Once the CNTs are sufficiently dispersed, CO2 is bubbled through the sample to create an acid-amine adduct using hydrogen bonding properties. This process allows the dispersing media to become viscous until heated, and then when heated, the solvents volatilize rapidly, leaving only the CNTs behind. However, this method does not generate long term stability. The acid-amine adduct made by the CO2 will degrade at room temperature, causing the viscosity to decrease with time. The adduct can be reformed by bubbling CO2 back through the sample, but this property necessitates onsite manufacturing, and the product must be used immediately after manufacture.
Similar fugitive viscosity modifiers include the use of solid ammonium acid salts as a viscosity modifier for organic bases. Ammonium carbonate is a salt composed of water, carbon dioxide, and ammonia. When a molecule that is sufficiently basic reacts with the ammonium carbonate or ammonium carbamate salt, it will displace the ammonia. If the molecule is dibasic, then a hydrogen bonded carbonic acid polymer chain can form. Since the fluid is then composed of long molecular chains, its viscosity will be increased dramatically. The hydrogen bonded polymer will decompose at a low temperature. The use of a solid reagent (ammonium carbonate) is simpler than the use of gaseous CO2 in the V2V™ process.
For transparent conductive applications, it is ideal that the carriers completely removable. Otherwise, residues may interfere with the transparency or conductivity of the traces. Many flexible substrates cannot tolerate high temperatures during post-processing, so it is important that the carrier can be easily removed at low temperatures.